The Other Sister
by IzzyMorgensternXX
Summary: Based on polandbananabooks theory that Isabelle Lightwood isn't completely a lightwood because of her eye colour. Who are Izzy's parents and what implications would it have if she was suddenly told the truth? Post COHF, Post J&T's wedding. Lots and lots of lovely Sizzy. Rated T for angst, normal Shadowhunter violence, possible tragedy, and making out. R&R please.
1. Chapter 1

I tangled my fingers into his long brown hair, and placed a kiss on his cheek as he sleep soundlessly beside me in bed.

The early dawn sunlight crept through my black curtains, like a star glowing in the eerie darkness of space- _gosh everything reminded me of Star Wars these days!_

I smiled as I rested my head back on my pink pillows, my long raven hair splayed out all around me, a strand brushing against Simon's hand. He still didn't remember everything- but at least he remembered everything about us.

"_I am Lord Montgomery, by the angel! Let me into the NY Institute!" Simon shrieked one night, demon ichor staining his Shadowhunter gear. Thank the angel that he was wearing glamour runes or else this would have been pretty embarrassing._

_I laughed in reply, "You remember?" _

"_Yes," Simon replied, blushing a beetroot red, "I can't believe Maureen dressed me in that shirt and those leather trousers."_

_We dissolved into laughter as his arm slid around my waist and pulled me towards him so that we could feel each other's breath on our faces._

_A loud cough broke us out of our role play. Alec- of course! I was going to have to say something about his timing. _

_Behind him stood Jace- a smug look on his face- and Clary- smiling but also trying to hide her second hand embarrassment at seeing her parabati being caught again by Alec._

I smiled to myself as I remembered. Simon could be geeky- and his D&D discussions at clave meetings were always amusing if a little annoying- but I loved him. I still hadn't told him yet but every morning I would wake up early just so I could whisper it to him before he woke. I knew that deep down I hoped one of those mornings he would wake and hear what I said- but I knew I had to face my fears; I had to tell him soon.

"I love you," I whispered gently, kissing his lips.

The kiss woke him up, he looked surprised- his eyes wide- but soon calmed down when he saw me.

"G' morning Izzy," Simon mumbled, "Isn't it usually the prince waking the princess not the other way around?"

I shrugged, "I thought we were the shark and the shark hunter?"

"Yes, but which one of us is the shark?" Simon asked.

"I most certainly am," I replied, pouncing on top of Simon so that my hair hung like a curtain around our faces.

A loud knock came from my bedroom door.

"I don't want to know what you're doing in there, and if it's illegal I'm going to castrate you Simon Nightraven!" An irritatingly familiar voice began,** "**Just get dressed Mum wants to talk to you about something."

"Ok!" I yelled back grabbing a boot and throwing it at the door, "Be there in five and there better be food."

_Break-Break-Break-Break-Break-Break-Break-Break-Break-Break-Break-Break-Break_

I tucked into a jam donut ravenously as Simon joined me on the couch in the living room. In the corner Church sat licking himself, the constant Persian cat look of disgust on his face- Zachariah was back in New York with Tessa to do some shopping and meet up with some of his old Silent Brother Buddies, consequently Church ended up back the institute.

"What do you want to talk to us about?" I said between mouthfuls of dough.

"There's something I meant to tell you a while ago," my Mum began, lacing and unlacing her long delicate pale fingers, her posture was perfect, her raven hair swept back into a meticulous high bun. Her look screamed authority- perfect for her job as the head of the NY institute.

"Spill," I replied. I was puzzled though- surely Mum had told me too much already, ruining my trust in men by telling me about Dad's affair- surely there wasn't anything else she could tell me.

"When you were little, do you remember how you asked me why you had had brown eyes, whilst your father, brothers and I had blue eyes? And I told you it was because something very special had happened when you were born that made you different?"

"Mum," I began, feeling uneasy; "I'm not six. I get it- you mean there's some genetic thing that's given me weird coloured eyes."

I watched as Simon shifted in his seat clearly uncomfortable.

"Simon, dear," my mother said, "Why don't you go see where Jace and Alec are, I think they said something about training?"

"Ok," Simon replied, reluctantly leaving, it was an obvious dismissal but he was hesitant leave me alone.

I stared at my Mum across the room, waiting for her to speak, the silence deafening, "You are right that it is genetics."

"What do you mean?"

"You must know from Hodge's lessons that it is very unusual for two blue eyed parents to have a brown eyed child- in fact it is very rare and always a genetic mutation. Eye colour is more complex than punnet squares lead us to believe. It's to do with pigments- someone with darker eyes passes on more of the darker pigment to their child, whereas someone with light eyes passes on more of the lighter pigment to their child. Most children have an eye colour somewhere in between that range- like Alec- or at the extreme of the range- like Max did with his grey eyes."

I internally winced at my little brother's name, "What are you trying to say?"

"You are not Robert's child," my mother said finally.

I freaked out, my breathing going fast, my heart pounding, the blood draining out of my pale face. _Alec and Max- they weren't completely my brothers- what, no! - That couldn't be true- it mustn't be true- it mustn't be true- it mustn't._

I felt my eyes sting and fill with tears, and a wave of nausea role in my stomach. My mother passed me a waste paper bin, which I immediately threw up into. My Mum sat next to me, brushing my hair away from my face. I nodded in thanks.

"Who is my father?" I asked when I was finished.

"Your father wasn't very kind," my mother began slowly, "I knew him from the Circle. I loved him dearly, and I thought he loved me too. But I found out later he'd said that to many women. He was ambitious and charming and I thought we had so much in common in that respect. I thought Robert held me back, humbled me- I thought I could have a much better life with your father- but I was wrong of course. He loved his wife too dearly to ever leave her, in fact he was deeply guilty about our little affair- he told me never to mention it to anyone. After the uprising, he was gone, presumed dead and I knew it was best to keep the truth a secret."

I felt shock spread through me. _The man she mentioned- he couldn't be, could he?- NO, NO, NO- I screamed mentally- my dark eyes, he had dark eyes- but so did his son- what if I was like his son too…._

I looked up at her pleadingly, wishing with all my will that my father was some long lost brother of a Carstairs or Herondale or Wayland something like that, not him, anyone but him.

"Izzy- your father was Valentine Morgenstern. Your name is not Isabelle Sophia Lightwood. It's Isabelle Serephina Morgenstern."


	2. Chapter 2

I knocked on the front door, barely able to see through the tears in my eyes. The trip on the Metro had flown by in a blur as I tried to hold in my emotions. _This just couldn't be true- surely there must have been some mistake- maybe my Mum was wrong._

No, it was true.

I took a shaky breath.

I wasn't sure if I should be there. But someone had to tell Clary, and I needed her now. I needed my _sister_. I was happy at how easy it was to think of Clary as my sister. But as for Sebastian- no, he would _never_ be my brother- not after everything that happened to Max.

I felt a tear slide down my cheek. After Mum had told me I had run straight out of the institute, my Mum following me, yelling that I should get back inside- that I should hear the rest of what she had to say.

The door opened to show Jocelyn. She smiled when she saw me but her expression changed as she saw me crying.

"Isabelle," she started, holding her arms towards me- offering a hug. I felt sick- she should hate me, she should absolutely hate me. I was awful, an awful demon child.

"Isabelle, what's wrong?" Jocelyn asked, "You can tell me. It's ok. Everything will be ok."

I laughed miserably, shaking my head, strands of hair sticking to my wet face, "It will never ok. Nothing will ever be ok."

"Should I get Clary?" Jocelyn asked; worry pulling at her face, as she gestured for me to go inside.

Clary came running down the stairs moments later, red curls bouncing on her shoulders. She wore clothes that would have normally made me make a comment about her lack of fashion sense.

Clary expression showed sympathy as she hugged me, "What's wrong, Izzy? Did Simon do something, he might be my parabati but I swear if-"

I hugged Clary back, feeling guilty for doing so, she should hate me too, "No," I replied, "I need to talk to you and your mum about something."

"Ok," Clary said confused, "Mum?"

"Yes?" Jocelyn said looking around the living room door.

"Izzy wants to speak to us about something," Clary replied, and shrugged when Jocelyn gave her a questioning look.

Jocelyn took a seat next to Clary on the sofa facing where I was sitting.

I started shakily, "I…I…I don't know what to do."

I buried my face in my hands, allow myself to cry.

"Izzy, have you told Simon?" Clary asked.

I laughed slightly, "It's not_ that_, Clary."

Jocelyn looked relieved- and was that a little disappointed? Clary just smiled. Both of them seemed to be oblivious about the gravity of the information I was about to tell them.

"It's about my Dad," I began, earning questioning looks, "Mum says my Dad's not Robert."

Clary looked shocked, but Jocelyn didn't look all that surprised.

Noticing the look I was giving her, Jocelyn replied, "I always had my doubts. I didn't want to think Maryse had cheated on him- but I guess it's pretty obvious that she did."

"Who's your father? Do you know?" Clary asked, breathing quickly, as she sat next to me on the sofa, hugging me.

I nodded, tears falling, "V…Valentine."

The silence seemed to go on forever. I cried even more furiously. I felt sorry for Jocelyn- knowing that her husband had cheated on her must be difficult- even with everything Valentine had done.

"Oh, Izzy," Clary said, hugging me so hard I struggled to breathe, "We're sisters!"

I laughed, "You were already a sister to me."

I noticed Jocelyn had moved to sit next to me.

"Isabelle," she replied, "I know this must be hard for you, but you need to tell Alec and Jace. Do they know yet?"

"I-" I began.

"Yes," Clary answered, "I got messages from both of them this morning. They were so worried after they saw you running off."

I cried more, I shouldn't have worried them. I didn't need my eyes to tell me that I was a demon child.

"And what about Simon?" I asked, looking at my sister through tear stained eyes.

"He left me ten texts and four voicemails so I guess so," Clary replied.

I smiled; at least I didn't have to worry about telling them, and seeing the disgust on their faces. I wouldn't be able to tell Max until I saw him in Idris later in the year.

I stopped smiling as I reminded myself that I was still a demon child, I instantly started crying again.

"Isabelle," Jocelyn, a hand on my shoulder, "Are you sure you've told us everything, dear? It seems like there's something else as well?"

"You should hate me," I replied in gasping breaths.

"You can't help who your parents are," she replied, looking at me with sympathy_. I was such a manipulative demon child._

"You should," I answered, "I'm evil."

"Oh, Izzy," Clary said, rubbing my back, "How could you ever think that? You spend all your time looking after me and Simon and your brothers, you'd never hurt anyone."

"By the angel, Clary!" I nearly shouted, making Jocelyn flinch, "I'm a demon, I have demon blood. My eyes are black. I'm clearly a demon child."

I began crying uncontrollably as Jocelyn and Clary tried to reassure me, telling me how good and kind I was. _What if I'd just manipulated everyone into thinking that?_

"You have dark brown eyes flecked with gold," said a voice coming from behind me, "Remember I wrote a song about it."

"Simon," I whispered quietly.

"Isabelle," Simon said, kneeling on the carpet in front of me, "You are not a Morgenstern, you _never_ will be a Morgenstern. But you could be a Nightraven if you wanted to? Isabelle, will you marry me?"


	3. Chapter 3

"I am not wearing that to Magnus's!" Clary yelled, looking at the sparkly blue skin tight dress was wearing with disgust.

"No, you're not," I replied, smiling at my sister, "You're wearing that to my fiancés D&D night. You're going as a fire breathing warlock-dragon hybrid."

"Well, this screams something far from warlock-dragon to me," Clary replied, trying unsuccessfully to pull the skirt of the dress down, "Why are all your dresses so short and tight?"

I heard a laugh from outside my bedroom door.

"Jace!" I yelled, as I rummaged through my wardrobe, trying to find a dress to wear to Magnus's after D&D.

Jace strolled into my room, not bothering to ask if it was ok to come in, and looked at Clary in a way that made me want to through my high heels at him.

"Hello, Izzy," Jace said chirpily- well as chirpy as Jace is.

"Your face is a very open pornographic book," I answered, earning a laugh.

"I think Alec said something like that in Edom," Jace contemplated, "Which is pretty rich considering how many times he's 'fallen on in neck'?"

Clary and I laughed.

"Yeah, I think even Church noticed," Clary added, patting the Persian fluff ball's stomach whilst he pretended to be dead. Someone ought to tell the cat that he was immortal and no one was falling for it.

"I can't believe you're going to D&D, Izzy," Jace said, a smirk on his face, "It's not the sort of thing I expected partners to do. I thought you would have already started planning the wedding."

"Easy now," another voice added- Alec, "She's planning mine and Magnus's first- don't you remember all those postcards she sent when we went travelling."

Jace laughed, "Did you really ask Magnus to give you $150,000 for diamanté crowns, a pony, lollipops and _your _dress?"

"Yup, she did," Alec answered.

"You're forgetting something," I added, pulling a short black dress out of my wardrobe, "You _are_ married, well at least according to the certificate, Church and I created."

Alec laughed, "Yes, Church was your lawyer and he's never lost a case."

I blushed, "That was ages ago, Alec. I was mad at Magnus for breaking up with you."

"Mad," Clary answered, "Doesn't begin to cover it. You threatened to break Magnus's door down, insulted him in front of his neighbours and told him no Lightwood would ever so much as wink at him a bar on the moon."

"And you said the Lightwoods are hotter than Herondales," Jace added, "Which is _totally_ out of order."

"Shadowhunter business- fighting demon and looking fine," Clary said, "And business is booming."

We all laughed. But I stopped short, should I still consider myself a Lightwood even after everything? I guessed in a few months it wouldn't matter.

-Break-Break-Break-Break-Break-Break-Break-Break-Break-Break-Break-Break-Break-

A knock on my bedroom door caused the pounding in my head to escalate.

"Come in!" I yelled wincing.

"Is my little sister feeling ok?" Alec asked, trying to show mock sympathy, but coming over a little too concerned.

I groaned, as Alec sat next to me on my bed and pulled me into his arms.

"I don't think you should have drunk so much," Alec began, "Though you don't usually get this hung-over after a party at Magnus's."

"I dunno," I shrugged, "Maybe there was something in the drinks."

I felt my stomach turn, and I ran towards the bathroom, before throwing up in the sink.

"This isn't like you, Izzy," Alec said, pulling my hair away from face, "You never get this hung-over, I think you've got a stomach bug or something, you should rest, you look really pale."

"I'm fine," I answered, wiping my face with tissue, "I'm hungry now, do you think Jace has left us any of those bagels?"

Alec looked at me in disbelief, muttering something about girls under his breath.

I didn't care, I was still confused about what had just happened.


	4. Chapter 4

I couldn't believe it.

My whole body was filled with dread and excitement, creating a buzzing feeling throughout my body that made it impossible to sit still for more than a second.

I paced nervously backwards and forwards across my room, dodging the clothes and makeup littering the floor. Every few steps I would pull my phone out of my cardigan pocket and check my inbox. It was stupid- I would hear if I'd received a text.

I heard my phone make a loud pinging noise, announcing that Simon had arrived at the Institute. I practically ran into the hallway- trying to burn off some nervous energy- and bumped into an infuriated Alec.

"How dare you, Herondale?!" Alec demanded, splotchy red marks covering his usually pale cheeks. His gaze was narrowed in on a cocky looking Jace standing at the other end of the hallway.

Jace leaned nonchalantly against a painting of the angel Raziel, he was turning the Herondale dagger over and over in his hands, and seemed mesmerised by the design on the handle.

All of a sudden he started humming a fast melody, and Alec became even more enraged.

"What's going on?" I asked, looking from one brother to the other.

"He is singing some ludicrous song he learnt from an ifrit at Taki's," Alec explained in a menacing cold tone.

"Who apparently learnt it from good old great-great grandpa Will," Jace added smugly, before singing, "Demon Pox, oh, Demon Pox, just how it is acquired, one must go down to the rough part of town, till one is very tired, oh, Demon Pox, oh, Demon Pox, I had it all along, no, not the Pox you stupid blocks I mean this very song!"

"Delightful," I replied, "What's demon pox?"

"It's basically an STD," Jace explained smiling, "Shadowhunters get it from improper relationships with demons."

"Ew," I answered grimacing, "Who would do that?"

Alec made to stop Jace from talking but was too late.

"You're ancestor, Benedict Lightworm," Jace replied.

"Lightworm?" I queried.

"Yup," Jace concluded, "Turned into a massive worm, and ate a load of people."

I rolled my eyes, "Only you would find that fascinating."

"What's fascinating?" I heard Simon ask as he appeared in the corridor.

"How long it took you to get here," Jace mused, "Izzy was going out of her mind waiting for you."

"Izzy, what's wrong?" Simon asked, concern clearly written on his face.

"I'm-" I began.

Then there was a loud bang, and suddenly everything went dark.


End file.
